1. Field
This invention relates to hydraulic relief valves and, more particularly, to pilot operated, balanced piston compound relief valves having replaceable modular venting valves for varying the characteristics of the relief valves.
2. Description of the Prior Art
Pilot operated, balanced piston compound relief valves are well known in the art for use in a variety of operations relating to hydraulic circuits. Such valves typically operate in two stages, the first stage comprising the opening of a pilot valve, and the second stage comprising the opening of a main valve which includes a balanced piston. Typically, the pilot valve is disposed in an upper portion of the relief valve, while the balanced piston is disposed in the main valve body which also includes the high flow port connections. The release pressure for the pilot in conventional relief valves can be pre-selected, and is preferably adjustable by the user.
The balanced piston includes a central bore formed therethrough, providing for fluid communication between the low pressure side of the pilot valve and a vent leading to the fluid reservoir (tank). The balanced piston also includes a shoulder portion having a longitudinally offset orifice for providing fluid communication between the high flow port connections of the main valve body and the high pressure side of the pilot valve. When the pressure within the main valve body exceeds the pilot valve setting, the pilot valve opens, venting fluid through the central bore of the piston to the tank. The release of pressure above the piston causes the higher pressure within the main valve body to force the piston upward against a light (20 PSI, for example) spring, thereby unseating the piston and allowing the high pressure fluid to vent directly from the main valve body to the tank.
It has been found advantageous to provide conventional relief valves with remote actuation means for effecting the venting of fluid through the main valve body in response to an input other than an increase in pressure sufficient to unseat the pilot valve. For example, it has been found desirable to modify the pilot valve housing to accept an electrically actuated solenoid valve, so that fluid can be vented through the central piston bore as described above in response to an electrical signal. In order to equip a conventional valve with such remote actuation means, however, it is necessary to substantially modify the pilot valve housing. Furthermore, different solenoids require different modification to the housing, so that a variety of solenoids may not be easily interchangeable with a single relief valve.
It has also been found desirable to incorporate a supplemental pilot valve onto the conventional pilot valve housing, the supplemental valve being either remotely actuatable or set to automatically vent at a pressure lower than that required for the primary pilot valve. A supplemental pilot valve may be added to the existing conventional pilot valve housing without making substantial modifications, but a secondary vent line must be added for venting the supplemental pilot valve to the tank.
A conventional pilot operated, balanced piston compound relief valve is shown in U.S. Pat. No. 3,613,717 entitled "Remote Electrically Modulated Relief Valve," issued Oct. 19, 1971. The '717 patent is illustrative of the above-mentioned problems, wherein the pilot valve housing requires substantial modification in order to incorporate an electrically actuated solenoid. Other conventional devices exemplifying the disadvantages present in prior art designs are shown in the catalogs of Fluid Controls, Inc., Vickers, Inc., and Nachi America.
In light of the disadvantages present in prior art apparatus as set forth above, a need has been recognized for an easily replaceable, remotely actuatable valve which can be incorporated into an existing pilot operated, balanced piston relief valve without requiring any significant modifications. A need has also been recognized for a pilot operated, balanced piston relief valve adapted to include a supplemental relief valve, ventable to the tank through the main valve body without necessitating additional vent lines.